Research On Electromagnetic Ion Cyclotron Waves Excited In A Plasma Containing Nitrogen Ions

Electromagnetic ion cyclotron wave is an important plasma wave in the inner magnetosphere.It is excited by the anisotropy of the velocity distribution of ions and can interact with the charged particles in space by resonance.It is accepted that space plasma contains electrons,hydrogen ions,helium ions and oxygen ions.Nitrogen ion content is small,and its mass is close to oxygen ion,so it is often ignored or regarded as oxygen ion.But a research suggests that this neglect is sometimes irrational.Plasma contains nitrogen ions,which will make the electromagnetic ion cyclotron wave appear many new characteristics,which will also affect the cyclotron resonance with the radiation belt particles,so it needs to be further studied.In this paper,the effects of magnetic field intensity,plasma number density,proportion of thermionic components and ion velocity distribution on the cyclotron dispersion and growth of electromagnetic ions in hot plasma containing nitrogen are systematically studied by using linear theory and satellite observation data.It is found that the dispersion of electromagnetic cyclotron waves can be changed from three wave bands to four wave bands by nitrogen ions.The increase of the proportion of hot hydrogen ions will promote the growth of hydrogen band waves,while the increase of hot nitrogen ions,hot oxygen ions and hot helium ions will inhibit the growth of hydrogen band waves.The increase of the anisotropy of hydrogen ions increases the growth rate of each wave band,while the increase of the anisotropy of heavy ions decreases the growth rate of hydrogen band wave.The increase of the parallel thermal velocity of hydrogen ions will strengthen the growth of each wave band,while the increase of the parallel thermal velocity of oxygen ions and nitrogen ions will inhibit the growth of hydrogen band waves.Two typical electromagnetic cyclotron events observed by RBSP probe were selected for linear theoretical analysis.The total proportion of various ionic components is calculated from the cut-off frequency,and the observed thermionic flux is fitted with the multi-sin^m-type distribution and the bi-Maxwell distribution,so as to obtain the accurate thermionic proportion,which is then applied to the linear theoretical calculation.This method makes up for the defect that the low-energy cold ions cannot be detected directly by the satellite,and the obtained wave frequency is in good agreement with the observed data.Finally,this paper studies the cyclotron resonance interaction between the electromagnetic cyclotron wave excited by a plasma containing nitrogen and the particles in radiation belt by combining the wave-particle resonance condition,the dipole magnetic field model and the ion number density model,and gives the variation of the minimum resonance energy E_min with the wave frequency and magnetic latitude.The results show that the minimum E_min of electron and hydrogen ions is of the order of Me V,while the minimum E_min of heavy ions can reach the order of e V or even lower.This is mainly because the heavy ion cyclotron frequency is in the electromagnetic cyclotron frequency range,while the electron and hydrogen ion cyclotron frequency is higher than the electromagnetic cyclotron frequency range.In this paper,the effects of various factors on EMIC waves excited by plasma containing nitrogen are studied comprehensively by using linear theory.With the background parameters in the satellite data,the growth rates of EMIC waves were calculated accurately.The minimum resonance energy E_min of all kinds of radiation belt particles is calculated by using the geomagnetic dipole model.These work contribute to the understanding of the excitation and growth of EMIC waves in space and their role in magnetosphere dynamics.